title: Molecular dissection of IZUMO1, a sperm protein essential for sperm-egg fusion.
abstract: Although the membrane fusion of spermatozoon and egg cells is the central event of fertilization, the underlying molecular mechanism remains virtually unknown. Gene disruption studies have showed that IZUMO1 on spermatozoon and CD9 on oocyte are essential transmembrane proteins in sperm-egg fusion. In this study, we dissected IZUMO1 protein to determine the domains that were required for the function of sperm-egg fusion. We found that a fragment of the N terminus (Asp5 to Leu113) interacts with fertilization inhibitory antibodies. It also binds to the egg surface and effectively inhibits fusion in vitro. We named this fragment 'IZUMO1 putative functional fragment (IZUMO1PFF)'. Surprisingly, IZUMO1PPF still maintains binding ability on the egg surface of Cd9(-/-) eggs. A series of biophysical measurements using circular dichroism, sedimentation equilibrium and small angle X-ray scattering revealed that IZUMO1PFF is composed of an N-terminal unfolded structure and a C-terminal ellipsoidal helix dimer. Egg binding and fusion inhibition were not observed in the IZUMO1PFF derivative, which was incapable of helix formation. These findings suggest that the formation of a helical dimer at the N-terminal region of IZUMO1 is required for its function. Cos-7 cells expressing the whole IZUMO1 molecule bound to eggs, and IZUMO1 accumulated at the interface between the two cells, but fusion was not observed. These observations suggest that IZUMO1 alone cannot promote sperm-egg membrane fusion, but it works as a factor that is related to the cellular surface interaction, such as the tethering of the membranes by a helical region corresponding to IZUMO1PFF-core.
authors.affiliation: Research Institute for Microbial Diseases, Osaka University, Yamadaoka 3-1, Suita, Osaka 565-0871, Japan.
authors.forename: Naokazu
authors.initials: N
authors.lastname: Inoue
authors.forename: Daizo
authors.initials: D
authors.lastname: Hamada
authors.forename: Hironari
authors.initials: H
authors.lastname: Kamikubo
authors.forename: Kunio
authors.initials: K
authors.lastname: Hirata
authors.forename: Mikio
authors.initials: M
authors.lastname: Kataoka
authors.forename: Masaki
authors.initials: M
authors.lastname: Yamamoto
authors.forename: Masahito
authors.initials: M
authors.lastname: Ikawa
authors.forename: Masaru
authors.initials: M
authors.lastname: Okabe
authors.forename: Yoshihisa
authors.initials: Y
authors.lastname: Hagihara
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal; D001665:Binding Sites; D055592:Biophysical Phenomena; D005260:Female; D007136:Immunoglobulins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D008297:Male; D008561:Membrane Fusion / Q000502:physiology; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008811:Mice, Inbred DBA; D018345:Mice, Knockout; D008822:Mice, Transgenic; D016297:Mutagenesis, Site-Directed; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000502:physiology; D020836:Protein Structure, Quaternary; D013084:Sperm-Ovum Interactions / Q000502:physiology*; D060245:Tetraspanin 29 / Q000172:deficiency / Q000235:genetics / Q000502:physiology
chemical_list: D000911:Antibodies, Monoclonal; C554870:Cd9 protein, mouse; D007136:Immunoglobulins; C498177:Izumo1 protein, mouse; D008565:Membrane Proteins; D010446:Peptide Fragments; D060245:Tetraspanin 29
keywords: IZUMO1; Membrane fusion; Sperm-egg fusion